U.S. patent number 5,883,176 [Application Number 08/863,929] was granted by the patent office on 1999-03-16 for conductive particles containing carbon black and processes for the preparation thereof.
This patent grant is currently assigned to Xerox Corporation. Invention is credited to John A. Creatura, Michael F. Cunningham, Thomas E. Enright, Paul J. Gerroir, Nancy Ann Listigovers.
United States Patent |
5,883,176 |
Gerroir , et al. |
March 16, 1999 |
Conductive particles containing carbon black and processes for the
preparation thereof
Abstract
Conductive polymeric particles can be formed by mixing a
monomer, carbon black and a block copolymer, wherein the block
copolymer contains an A block that is miscible with said monomer
and a B block that anchors to the surface of the carbon black, such
as polystyrene or a derivative of polystyrene. A polymerization
initiator is added to the mixture and bulk polymerization is
effected until about 5 to about 30 weight percent of the monomer
has been polymerized. This partially polymerized product is then
dispersed in water and further polymerized.
Inventors: |
Gerroir; Paul J. (Oakville,
CA), Listigovers; Nancy Ann (Oakville, CA),
Cunningham; Michael F. (Georgetown, CA), Enright;
Thomas E. (Whitby, CA), Creatura; John A.
(Ontario, NY) |
Assignee: |
Xerox Corporation (Stamford,
CT)
|
Family
ID: |
24315589 |
Appl.
No.: |
08/863,929 |
Filed: |
May 27, 1997 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
579107 |
Dec 27, 1995 |
5747577 |
|
|
|
Current U.S.
Class: |
524/458; 523/200;
523/201; 252/502; 252/511; 523/204 |
Current CPC
Class: |
H01B
1/24 (20130101); C08F 292/00 (20130101); C08F
287/00 (20130101) |
Current International
Class: |
C08F
287/00 (20060101); C08F 292/00 (20060101); H01B
1/24 (20060101); C08F 002/44 (); H01B 001/06 ();
G03G 009/113 () |
Field of
Search: |
;524/458
;523/200,201,204 ;252/511,502 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Seidleck; James J.
Assistant Examiner: Asinovsky; Olga
Attorney, Agent or Firm: Oliff & Berridge, PLC
Parent Case Text
This is a division of application Ser. No. 08/579,107 filed Dec.
27, 1995 now is U.S. Pat. No. 5,747,577
Claims
What is claimed is:
1. A conductive particle comprising a polymer matrix, carbon black,
and a block copolymer, wherein the block copolymer comprises an A
block which is miscible with the polymer matrix, and a B block,
which anchors to a surface of the carbon black.
2. The conductive particle of claim 1, wherein the particle has a
number average particle size of from 0.1 to 5 microns.
3. The conductive particle of claim 1, wherein the particle has a
number average particle size of from 0.1 to 1 micron.
4. A conductive carrier comprising carrier particles coated with at
least one conductive particle of claim 1.
5. A conductive particle formed by a process comprising:
(a) mixing at least one monomer, carbon black, and a block
copolymer, wherein the block copolymer comprises an A block, which
is miscible with a polymer of the at least one monomer, and a B
block, which anchors to a surface of the carbon black;
(b) adding a polymerization initiator to the mixture of step
(a);
(c) effecting bulk polymerization of the mixture of step (b) until
about 5 to about 30 weight percent of the at least one monomer has
been polymerized;
(d) dispersing the partially polymerized product in water; and
(e) polymerizing the resulting dispersion.
Description
BACKGROUND OF THE INVENTION
This invention is generally directed to conductive particles and
processes for the preparation thereof. More specifically, the
present invention relates to conductive particles containing a
polymer matrix and a carbon black dispersion, wherein the carbon
black is evenly distributed throughout the polymer matrix using a
block copolymer that contains a polymer block that is miscible with
the polymer matrix and a second block that anchors to the surface
of the carbon black.
In electrostatographic imaging and related development processes,
images are developed using a developer generally comprising colored
toner particles and carrier particles. Carrier cores generally
comprise metals, which are conductive or semiconductive materials.
Polymeric materials are generally used to coat the surface of the
metals and are usually insulating. Therefore, carrier particles
coated completely with polymer or a mixture of polymers can lose
their conductivity and become insulating. Although this is desired
for some applications, other applications require the carriers to
have specific conductivity properties. Conductive magnetic brush
development systems, for example, require carrier particles that
are conductive. Since the carrier polymer coating can also control
the triboelectric charging properties of the carrier, a conductive
carrier coating is needed to design carriers with the desired
conductivity and triboelectrical properties. However, traditional
conductive polymers are very costly, and are not suitable for
preparing low cost coatings. Thus, a conductive polymer composite
comprising a low cost polymer and a conductive filler, such as
conductive carbon black, is considered a more suitable
alternative.
The coating of metal materials, such as carrier beads, with a
polymer is known and can be achieved by two general approaches,
solution and powder coating. Solution coating of carriers using a
polymer composite solution comprised of a polymer, a conductive
filler and solvent can be utilized to prepare a conductive carrier.
However, trapping of solvent in the solution coating adversely
interferes with the use of the coated materials. For example,
residual solvent trapped in the carrier coating reduces the carrier
life and the release of the solvent in the developer housing can
cause other problems due to the harmful effects of adsorbed solvent
on various copying machine parts and the toxicity of solvent.
Moreover, the solvent recovery operation involved in solution
coating processes is costly.
The powder coating of metal surfaces such as the carrier cores can
eliminate the need for solvents and, therefore, many of the
problems associated with solution coating. However, powder coating
requires polymer powder that is very small in size, for example
less than one micron. Although several polymer powders with desired
particle size are available for carrier powder coating, there is a
need for very small conductive polymeric particles, particularly,
submicron particles, containing a conductive filler distributed
evenly throughout the particles.
The preparation of polymeric particles for powder coatings can be
accomplished primarily by three methods, namely grinding or
attrition, precipitation and in situ particle polymerization. In
grinding or attrition processes, especially fluid energy milling,
large polymeric particles or polymeric composite particles
containing fillers are reduced to the size needed for powder
coating, for example less than one micron. However, such processes
are often not desirable both from an economic and functional
viewpoint. These materials are difficult to grind, and therefore,
grinding or attrition of the required materials for coating with
present milling equipment is very costly due to very low processing
yield, for example in the range of 5 to 10 weight percent.
Precipitation processes can also be used to prepare polymeric and
polymeric composite particles. In one approach, the polymer
solution is heated to above its melting temperature and then cooled
to form particles. In another process, the polymer solution is
precipitated using a nonsolvent or the polymer solution is spray
dried to obtain polymeric and polymeric composite particles. With
all of these precipitation processes, it has been difficult to
achieve low cost and clean polymer particles, that is, for example,
with no or substantially no impurities such as solvents or
precipitants in the resulting polymer particles. Furthermore, it is
also difficult to obtain particles with small particle size and
narrow particle size distribution. It is also difficult to control
filler distribution throughout each particle's polymer matrix.
Suspension polymerization can be utilized to prepare polymer
particles and polymeric composite particles containing, for
example, a conductive filler. The main advantage of suspension
polymerization is that the product may easily be recovered.
Therefore, such a process is considered economical. However, it is
very difficult to prepare very small particles by suspension
polymerization, for example having a size less than five microns,
because the monomer droplets tend to coalesce during the
polymerization process, especially in the initial stage of
polymerization where the droplets are very sticky.
U.S. Pat. No. 4,835,084 discloses a method for preparing pigmented
particles wherein high concentrations of silica powder are used in
the aqueous phase to prevent coalescence of the particles. U.S.
Pat. No. 4,833,060 discloses a process for the preparation of
pigmented particles by dissolving polymer in a solvent that is
immiscible with water and dispersing the solution that is formed
thereby in an aqueous phase containing silica powder to prevent
coalescence of the particles. However, the silica powder used in
both of these processes is removed using KOH, which is costly, and
residual KOH and silica materials left on the surface of the
particles affect the charging properties of the particles.
Moreover, the above processes do not teach the preparation of
submicron conductive particles.
In in situ particle polymerization processes, polymer particles are
prepared by using suspension, dispersion, emulsion and
semisuspension polymerization processes. Although emulsion and
dispersion polymerization processes can be utilized to prepare
polymeric particles smaller than one micron by nucleation and
growth, these processes do not readily enable synthesis of
particles containing fillers such as conductive fillers. Conductive
fillers, such as carbon blacks, are free radical polymerization
inhibitors and thus tend to reduce the rate of polymerization in
such processes.
U.S. Pat. No. 5,043,404, the disclosure of which is totally
incorporated herein by reference, discloses a semisuspension
polymerization process for the preparation of small polymeric
particles that are comprised of a mixture of monomers or
comonomers, a polymerization initiator, a crosslinking component
and a chain transfer component, which are bulk polymerized until
partial polymerization is accomplished. The resultant partially
polymerized monomer or comonomers is dispersed in water containing
a stabilizer component with, for example, a high shear mixer. Then,
the resultant suspension is polymerized, followed by washing and
drying the submicron polymeric particles. However, the patent does
not disclose submicron conductive polymeric particles containing
fillers.
U.S. Pat. 5,236,629 discloses a process for the preparation of
submicron particles using the semisuspension polymerization
process. However, when carbon black is used as a conductive filler
with monomers that do not have a high affinity for the carbon
surface, particularly methacrylates and acrylates, the carbon black
dispersion tends to be poor because of the difficulty in (1)
dispersing the carbon black uniformly into the monomer initially
(prior to polymerization) and (2) maintaining a stable carbon black
dispersion (i.e. preventing formation of aggregates and clusters)
during polymerization. "Poor carbon black dispersion" means that
(1) some of the submicron particles contain very little or no
carbon black, and/or (2) the carbon black present in the particles
is not uniformly distributed within the particle, but rather is
present as clusters. Either condition results in lower conductivity
than is achieved when the carbon black is distributed uniformly
throughout the submicron polymeric particles.
U.S. Pat. No. 5,484,681 describes a process for the preparation of
submicron conductive particles that uses a diblock copolymer to
tailor the triboelectric charge of the particle. This is
accomplished by selecting the two blocks such that they diffuse to
the particle surface during polymerization and thus have a
significant effect on charge. However, the application does not
disclose improving carbon black dispersion.
Even or homogeneous distribution of fillers such as carbon black is
not believed achievable with the prior art processes mentioned
herein. In fact, in prior art processes, the conductive filler is
agglomerated around some of the polymeric particles and many of the
other polymeric particles contain little or no conductive filler.
Therefore a need exists for a invention to enable preparation of
submicron conductive particles with even carbon black
dispersion.
SUMMARY OF THE INVENTION
The present invention is directed to a process for the preparation
of conductive polymeric particles. The process comprises: (a)
mixing a monomer, carbon black, and a block copolymer comprising an
A block that is miscible with the monomer and a B block that
anchors to the surface of the carbon black; (b) adding a
polymerization initiator to the mixture; (c) effecting bulk
polymerization until about 5 to about 30 weight % of the monomer
has been polymerized; (d) dispersing the partially polymerized
product in water; and (e) polymerizing the resulting
dispersion.
The process of the present invention provides small, and preferably
submicron, conductive particles comprising a polymer matrix of the
monomer or monomers, carbon black, and the block copolymer. The
block copolymer acts as a dispersant/stabilizer for carbon black in
the monomer, and eventually in the polymer matrix, aiding the
breakdown of carbon black aggregates and preventing their
reflocculation. The A block of the block copolymer acts as a steric
stabilizer, preventing reflocculation of the carbon black
particles, and the B block acts as an anchoring group attaching
itself to the surface of the carbon black. As a result, the
polymeric particles display very uniform carbon black dispersion,
free of large aggregates and pigmentless particles. These
conductive particles can be used to form coated carrier
particles.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a micrograph acquired using a transmission electron
microscope demonstrating a carbon black dispersion according to the
present invention, as described in Example I.
FIG. 2 is a micrograph acquired using a transmission electron
microscope demonstrating a carbon black dispersion that does not
contain an AB block copolymer according to the present invention,
as described in Comparative Example I.
DETAILED DESCRIPTION OF THE INVENTION
Any monomer unit traditionally used to form semisuspension
polymerized conductive carrier powder can be used in the present
invention to form the polymer matrix. For example, suitable
monomers include, but are not limited to: monocarboxylic acids and
their derivatives such as acrylic acid, methyl acrylate, ethyl
acrylate, butyl acrylate, dodecyl acrylate, octyl acrylate, phenyl
acrylate, methacrylic acid, methyl methacrylate, ethyl
methacrylate, butyl methacrylate, octyl methacrylate, acrylonitrile
and acrylamide; dicarboxylic acids having a double bond and their
derivatives such as maleic acid, monobutyl maleate and
dibutylmaleate; vinyl esters such as vinyl chloride, vinyl acetate
and vinyl benzoate; vinyl ketones such as vinyl methyl ketone and
vinyl ether ketone; vinyl ethers such as vinyl ethyl ether and
vinyl isobutyl ether; vinyl naphthalene; unsaturated monoolefins
such as isobutylene and the like; vinylidene halides such as
vinylidene chloride and the like; N-vinyl compounds such as N-vinyl
pyrrole; fluorinated monomers such as allyl pentafluorobenzene and
the like; and mixtures thereof. The polymer matrix may contain more
than one monomer. In a preferred embodiment, the monomer unit is
methylmethacrylate.
The block copolymer comprises a polymer block (the A block) that is
miscible with the above monomer and with a polymer of the above
monomer. In a preferred embodiment of the present invention, the
polymer block is of the same monomer as the monomer forming the
polymer matrix. Use of the same monomer, or one that is miscible
with the polymer matrix, provides a "brush-like" steric stabilizing
structure around the carbon black particle, thereby stabilizing it
in the polymer matrix.
The block copolymer also comprises a B block that anchors to the
surface of the carbon black. The B block comprises monomer units
that have an affinity for the surface of the carbon black such that
the polymer block preferably adsorbs on the surface of the carbon
black rather than being solvated in the suspension. Thus, the B
block is selected based on the surface characteristics of the
carbon black being used. Generally, the B block is formed from
different monomers than the polymer matrix. In a preferred
embodiment of the present invention, the B block is immisible in
the polymer matrix-forming monomer and in the polymer matrix formed
from the monomer.
In an embodiment of the present invention, polystyrene and
polystyrene derivatives may be used as B blocks since they have an
affinity for many carbon blacks and are immisible in the A blocks
suggested above. Polystyrene derivatives that can be used in
embodiments of the present invention include, but are not limited
to, polychlorostyrene, polymethylstyrene, poly(t-butylstyrene), and
the like. For many carbon blacks, alternative B blocks include, but
are not limited to, poly(vinyl naphthalene), poly(4-vinylpyridine)
and other polymers of aromatic monomers, particularly non-polar
aromatic monomers.
In the present invention, it is preferred that the block copolymer
be an AB block copolymer. However, other suitable block polymers,
such as triblock copolymers, may suitably be used as long as the
objects of the present invention are achieved.
The block copolymer can be prepared by any known means for
preparing block copolymers, for example, such as ionic
polymerization or group transfer polymerization. Such processes are
described in Encyclopedia of Polymer Science and Engineering,
Volume 2, page 324, John Wiley and Sons, New York, 1984, the
disclosure of which is totally incorporated herein by
reference.
In an embodiment of the present invention, the block copolymer
contains from about 70 to about 98 parts by weight of the A block
and from about 2 to about 30 parts by weight of the B block per 100
parts by weight block copolymer. In a preferred embodiment, the
block copolymer contains from about 80 to about 92 parts by weight
of the A block and from about 8 to about 20 parts by weight of the
B block per 100 parts by weight block copolymer.
In an embodiment of the present invention, the number average
molecular weight of the A block is from about 5,000 to about
50,000, and preferably from about 10,000 to about 30,000. In a
further embodiment of the present invention, the number average
molecular weight of said B block is from about 800 to about 6,000,
and preferably from about 1,200 to about 4,000.
The amount of block copolymer mixed with the polymer matrix-forming
monomer is generally from about 0.1 to about 10 parts by weight per
100 parts by weight of monomer. In a preferred embodiment, the
amount of block copolymer is from about 0.5 to about 5 parts by
weight per 100 parts by weight of monomer.
Any of the various conductive carbon blacks known in the art may be
used in the present invention. Examples of suitable carbon black
include, but are not limited to, lamp black, furnace black,
acetylene black (available from Chevron Chemical), VULCAN
BLACK.TM., BLACK PEARL L.RTM., KEYTJEN BLACK EC600JD.RTM.
(available from Akzo), and CONDUCTEX SC ULTRA.TM. (available from
Columbian Chemical).
The amount of carbon black mixed with the polymer matrix-forming
monomer is generally from about 1 to about 80 parts by weight per
100 parts by weight of monomer. In a preferred embodiment, the
amount of carbon black is from about 5 to about 30 parts by weight
per 100 parts by weight of monomer.
The polymer matrix-forming monomer, the carbon black and the block
copolymer can be mixed together in any order as long as the
resultant mixture contains all three components before the
polymerization initiator is added. The components may be mixed by
any known process, such as by milling the components in a ball-mill
for, for example, 8-15 hours.
Any of the various polymerization initiators known in the art can
be used in the process of the present invention. Examples of the
polymerization initiator include, but are not limited to, azo-type
initiators such as 2,2'-azodimethylvaleronitrile,
2,2'-azoisobutyronitrile, azobiscyclohexanenitrile,
2-methylbutyronitrile, and the like; peroxide type initiators such
as benzoyl peroxide, lauryl peroxide,
1,1-(t-butylperoxy)-3,3,5-trimethylcyclohexane,
n-butyl-4,4-di-(t-butylperoxy)valerate, dicumyl peroxide, and the
like; mixtures thereof and the like. Further, benzoyl peroxide is a
preferred polymerization initiator because it helps to maintain the
dispersion.
In a preferred embodiment of the present invention more than one
polymerization initiator may be used. For example, a combination of
polymerization initiators that initiate polymerization at different
temperatures may be used. This enables a greater amount of the
monomer to be polymerized.
The amount of polymerization initiator mixed with the monomer
mixture is generally from about 0.1 to about 5 parts by weight per
100 parts by weight of monomer. In a preferred embodiment, the
amount of polymerization initiator is from about 0.5 to about 2.5
parts by weight per 100 parts by weight of monomer.
In a preferred embodiment of the present invention, a crosslinking
agent is added to the monomer mixture before or after the addition
of the polymerization initiator. Any of the various known
crosslinking agents may suitably be used. The crosslinking agent
may have two or more polymerizable double bonds. Examples of
suitable crosslinking agents include, but are not limited to,
aromatic divinyl compounds such as divinylbenzene and
divinylnapththalene; carboxylic acid esters having two double bonds
such as ethylene glycol diacrylate, ethylene glycol
dimethylacrylate and the like; divinyl compounds such as divinyl
ether, divinyl sulfite, divinyl sulfone and the like; mixtures
thereof; and the like. Among these, divinylbenzene is
preferred.
The amount of crosslinking agent mixed with the monomer mixture is
generally from about 0.01 to about 5 parts by weight per 100 parts
by weight of monomer. In a preferred embodiment, the amount of
crosslinking agent is from about 0.1 to about 1 parts by weight per
100 parts by weight of monomer.
In a further preferred embodiment of the present invention, a chain
transfer agent may be added to the mixture before or after the
addition of the polymerization initiator to control the molecular
weight by inhibiting chain growth Chain transfer agents that may be
used in embodiments of the present invention include, but are not
limited to, mercaptans such as laurylmercaptan, butylmercaptan and
the like, halogenated carbons such as carbon tetrachloride or
carbon tetrabromide and the like, and mixtures thereof. The chain
transfer agent is preferably used in an amount of from about 0.01
to about 1 part by weight per 100 parts by weight of monomer.
After the polymerization initiator and the optional crosslinking
agent and chain transfer agent are added to the monomer mixture,
bulk polymerization is effected until about 5 to about 30 wt. %,
preferably about 10 to about 20 wt. %, and more preferably about 10
to about 15 wt. %, of the monomer has been polymerize.
After the partial polymerization has occurred, the bulk
polymerization is preferably halted, for example by decreasing the
reaction temperature. The partially polymerized product is
dispersed in water to form a dispersed organic phase. The amount of
water used is generally 2 to 5 times the volume of the dispersed
partially polymerized organic phase. The water may optionally
contain a small amount of stabilizer in order to assure that the
partially polymerized organic phase is adequately dispersed in the
water. For example, the water may contain from about 0.1 to about 5
wt. % stabilizer.
Examples of suitable stabilizers include, but are not limited to,
nonionic and ionic water-soluble polymeric stabilizers such as
methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, block
copolymer such as PLURONIC E87.TM. (available from BASF), sodium
salt of carboxyl methyl cellulose, polyacrylate acids, salts
therefore and the like; polyvinyl alcohol, gelatins, starches,
gums, alginates, zein, casein and the like; barrier stabilizers
such as tricalcium phosphate, talc, barium sulfate and the like;
mixtures thereof and the like. Among these, polyvinyl alcohol with
a weight average molecular weight of from about 1,000 to about
10,000 is particularly preferred.
In a preferred embodiment of the present invention, an aqueous
phase polymerization inhibitor, such as potassium iodide, is then
added to the solution to minimize formation of emulsion polymer. A
microsuspension is then formed by, for example, homogenizing the
solution. The resulting dispersion is then further polymerized in
suspension by heating the aforementioned microsuspension to a
temperature of between 40.degree. C. and 120.degree. C., and more
preferably between 60.degree. C. and 90.degree. C., for a period of
2-12 hours. During this polymerization in suspension, the reactants
may be agitated with, for example, a conventional turbine agitator
to obtain the conductive particles of the present invention. After
the particles are formed, they may be washed and dried by, for
example, diluting the polymeric solution with deionized water,
centrifuging the solution, and removing the supernatant.
The conductive particles of the present invention generally have a
number average particle size (average diameter) of from about 0.1
to about 5 microns. In a preferred embodiment of the presemt
invention the particles have a number average particle size of from
about 0.1 to about 1 micron. In a further preferred embodiment of
the present invention, the particles have a number average particle
size of from about 0.1 to about 0.5 micron. In addition, carbon
black is evenly distributed among the particles and within each
particle. Further, only a few of the particles, if any, do not
contain carbon black.
The conductive particles of the present invention generally have a
number average molecular weight (as determined by gel permeation
chromatography) of from about 10,000 to about 250,000. In a
preferred embodiment of the present invention the particles have a
number average molecular weight of from about 20,000 to about
50,000. The conductive particles of the present invention generally
have a weight average molecular weight (as determined by gel
permeation chromatography) of from about 30,000 to about 2,500,000.
In a preferred embodiment of the present invention the particles
have a weight average molecular weight of from about 100,000 to
about 1,000,000. The conductive particles of the present invention
generally have a resistivity of from about 1 to about 500 ohm.cm.
In a preferred embodiment of the present invention the particles
have a resistivity of from about 10 to about 200 ohm.cm.
These conductive particles may be coated on carrier cores to form
coated conductive carrier particles. In an embodiment of the
present invention, about 100 grams of core carrier may be mixed
with about one gram of the conductive particles in, for example, a
Munson type mixer. The conductive particles are then fused to the
surface of the core carrier using, for example, a rotary kiln
furnace. Coated carrier particles formed by this process have
increased conductivity compared to those formed without the block
copolymer.
The process and the conductive particles of the present invention
are further defined by reference to the following illustrative
examples, it being understood that the invention is not limited to
the materials, conditions, process parameters, etc. recited herein.
All parts and percentages are by weight unless otherwise
indicated.
EXAMPLE I
Methylmethacrylate monomer (200 grams) and carbon black (40 grams)
are milled in a ballmill containing stainless steel grinding media
for 2 hours. A poly(methyl methacrylate)-b-polystyrene block
copolymer (4.0 grams) is then added to the ball mill and milling is
continued for 10 hours. This block copolymer is 20 weight percent
polystyrene and 80 weight percent poly(methyl methacrylate). The
number average molecular weights of the polystyrene and poly(methyl
methacrylate) blocks are 4,000 and 20,000 respectively.
2,2'-azobis(2,4-dimethylvaleronitrile) (3.0 grams), benzoyl
peroxide (1.0 gram) and divinyl benzene crosslinking agent (0.85
gram) are then mixed into the methyl methacrylate/carbon
black/block copolymer slurry for one hour in a one liter
reactor.
This mixture is bulk polymerized by heating to 45.degree. C. until
10 weight percent of the monomer, as measured by gravimetry, is
converted to polymer. The bulk polymerization is quenched by
cooling the reactor. The reactor contents, hereafter referred to as
the organic phase, is then poured into a 2L steel beaker, along
with 600 grams of an aqueous solution of 2.5 weight percent
polyvinyl alcohol having a weight average molecular weight of
3,000. The resulting mixture is homogenized for 5 minutes to
produce a microsuspension of polymeric particles containing carbon
black in water. Potassium iodide (5.0 grams) is then added as an
aqueous phase polymerization inhibitor.
The resulting microsuspension is transferred to a one liter
stainless steel reactor and the temperature is raised from
25.degree. to 60.degree. C. in 35 minutes where it is held for 2
hours; the temperature is then increased to 85.degree. C. during a
2 hour period and held there for 1 hour, after which the suspension
is cooled in 30 minutes to 25.degree. C. When cooled to 25.degree.
C. the suspension polymerization is complete as measured using gas
chromatography. The microsuspension product is then diluted with 1
L of deionized water and centrifuged. The supernatant is discarded;
the wet cake is diluted with 1 L of water and again centrifuged.
Again the supernatant is discarded and the wet cake is freeze
dried.
Scanning electron microscope photomicrographs of the dry product
show that the average particle size of the polymer product is 0.8
micron. Transmission electron microscope micrographs, as
demonstrated in FIG. 1, show good carbon black dispersion in which
the carbon black is uniformly dispersed within individual particles
and few particles do not contain carbon black. The glass transition
temperature is 110.degree. C., as measured by DSC. The product
resistivity, measured by pressing a pellet of the dry product, is
75 ohm.cm.
The resulting poly(methyl methacrylate) particles (0.9 grams)
containing carbon black with block copolymer are mixed with 100
grams of core carrier with an average bead diameter of 90 microns
in a Munson type mixer at room temperature. The coated materials
are then fused on the surface of the carrier at 400.degree. C. in a
rotary kiln furnace. Functional evaluation of the resulting carrier
in a two component development system shows that the carrier has a
conductivity of 10.sup.-6 (ohm.cm).sup.-1.
EXAMPLE II
Polymeric particles are formed as in Example I, except that 20
grams of the carbon black and 2.0 grams of the block copolymer are
used. In addition, the mixture is bulk polymerized until 13 weight
percent of the monomers, as measured by gravimetry, is converted to
polymer.
Scanning electron microscope photomicrographs of the dry product
show that the average particle size of the polymer product is 0.7
micron. Transmission electron microscope micrographs show good
carbon black dispersion in which the carbon black is uniformly
dispersed within individual particles and few particles do not
contain carbon black. The glass transition temperature is
109.degree. C., as measured by DSC. The product resistivity,
measured by pressing a pellet of the dry product, is 167
ohm.cm.
The resulting poly(methyl methacrylate) particles (0.9 grams)
containing carbon black with block copolymer are mixed with 100
grams of core carrier with an average bead diameter of 90 microns
in a Munson type mixer at room temperature. The coated materials
are then fused on the surface of the carrier at 400.degree. C. in a
rotary kiln furnace. Functional evaluation of the resulting carrier
in a two component development system shows that the carrier has a
conductivity of 10.sup.-7 (ohm.cm).sup.-1.
EXAMPLE III
Polymeric particles are formed as in Example I, except that the
block copolymer is 10 weight percent polystyrene and 90 weight
percent poly(methyl methacrylate) and the number average molecular
weights of the polystyrene and poly(methyl methacrylate) blocks are
2,000 and 18,000, respectively. In addition, the mixture is bulk
polymerized until 11 weight percent of the monomer, as measured by
gravimetry, is converted to polymer.
Scanning electron microscope photomicrographs of the dry product
show that the average particle size of the polymer product is 0.8
micron. Transmission electron microscope micrographs show good
carbon black dispersion in which the carbon black is uniformly
dispersed within individual particles and few particles do not
contain carbon black. The glass transition temperature is
109.degree. C., as measured by DSC. The product resistivity,
measured by pressing a pellet of the dry product, is 69 ohm.cm.
The resulting poly(methyl methacrylate) particles (0.9 grams)
containing carbon black with block copolymer are mixed with 100
grams of core carrier with an average bead diameter of 90 microns
in a Munson type mixer at room temperature. The coated materials
are then fused on the surface of the carrier at 400.degree. C. in a
rotary kiln furnace. Functional evaluation of the resulting carrier
in a two component development system shows that the carrier has a
conductivity of 10.sup.-6 (ohm.cm).sup.-1.
Comparative Example I
Example I is repeated except that the poly(methyl
methacrylate)b-polystyrene block copolymer is not added. Scanning
electron microscope photomicrographs of the dry product show that
the average particle size of the polymer product is 0.9 micron.
Transmission electron microscope micrographs, as demonstrated in
FIG. 2, show very poor carbon black dispersion, in which many
particles do not contain carbon black and the carbon black tends to
be in the form of large clumps or aggregates where it is present.
The glass transition temperature is 110.degree. C., as measured by
DSC. The product resistivity, measured by pressing a pellet of the
dry product, is 1300 ohm.cm.
The resulting poly(methyl methacrylate) particles (0.9 grams)
containing carbon black is mixed with 100 grams of core carrier
with an average bead diameter of 90 microns in a Munson type mixer
at room temperature. The coated materials are then fused on the
surface of the carrier at 400.degree. F. in a rotary kiln furnace.
Functional evaluation of the resulting carrier in a two component
development system shows that the carrier has a conductivity of
10.sup.-10 (ohm.cm).sup.-1, as compared to 10.sup.-6
(ohm.cm).sup.-1 when the block copolymer is used (Example 1).
* * * * *